A method and a device for coating a substrate by means of cathode sputtering is known, e.g., from WO 03/071579. Said document discloses a sputtering cathode by means of which a coating of a substrate can be produced in particular by using magnetic and/or magnetizable materials. The sputtering cathode comprises a cathode base body, a target arranged thereon and a magnet arrangement arranged behind the target. In order to be able to align the light magnetization axis of the material to be applied, the sputtering cathode further comprises means for generating an external magnetic field having magnetic field lines extending substantially parallel in the plane of the substrate. The cathode is preferably a long cathode having a length corresponding to at least a diameter of the substrate. For achieving a homogeneous coating, the substrate to be coated is moved during cathode sputtering below the long cathode in a direction transversely with respect to the longitudinal side of the long cathode in the substrate plane.
In order to achieve specific layer properties when using thin layers applied in this manner, a very particular composition of the deposited layer might be necessary.
Examples of such layers are Ni(1−x)Fex, wherein x=0.18 . . . 0.45, or Pt(1−x)Mnx, wherein x≈0.5, wherein specific properties of the layers depend on the exact value x. For producing layers of this kind, targets consisting of the desired ratio of the alloy to be applied are used.
However, in the field of research and development it is in most cases unknown which alloy composition of a specific layer leads to the best results for the specific purpose. It is a further problem—mainly in case materials having clearly different atomic masses are used—that during cathode sputtering the alloy composition can change during the deposition. The alloy composition of the layer deposited on the substrate thus can be more or less changed as compared to the alloy composition in the sputtering target, e.g. when using PtMn. Moreover, this change depends on the parameters of the coating process, e.g., the process pressure, the coating rate, etc. Since in the end the composition of the alloy on the substrate is important, it is often difficult to determine the optimum composition of the alloy of the sputtering target.
An often used but very inflexible method for adjusting alloys of thin layers that are deposited by means of cathode sputtering is based on the use of a series of sputtering targets with different alloy compositions that may be very close to one another. This method is very costly, in particular in case expensive materials (PtMn, FePt, etc.) are used. A plurality of targets with corresponding alloys must be produced and/or bought, wherein after corresponding series tests using the different targets, in the end only one target thereof comes close to the optimum alloy and can actually be used.
In these series tests, the corresponding targets must be inserted in and removed from the corresponding vacuum systems, which is very time-consuming.
Adjusting the alloy composition of alloy layers on a substrate by means of the coating process itself, i.e. by means of externally adjustable changes to the coating process and/or the coating geometry is, as compared thereto, a clearly simpler, faster and as a rule also more cost-efficient method.
A method that is very often used in this regard is the so-called co-sputtering. According to this method, the substrate is typically coated simultaneously by two (or more) sputtering cathodes each using different target materials. By changing the power ratio of both cathodes, also the ratio of the two material amounts, i.e. the alloy composition, on the substrate is changed. Mainly when using relatively large substrates it is difficult in connection with this technology to produce a homogeneous alloy on the substrate and at the same time achieve a homogeneous layer thickness. For improving the layer thickness homogeneity, the substrate is rotated during the deposition.
In accordance with another method, multiple layers of the two (or also more than two) materials are alternatingly applied to the substrate and then—typically in the vacuum—exposed to a suitably high temperature (“tempering”). This is said to cause a diffusion process leading to a mixing or homogenization of the materials and thus finally producing the desired alloy. The alloy composition is adjusted by selecting the primary layer thicknesses. For this method to function it is necessary that the diffusion process can be carried out at suitable temperatures and in a reasonably short time. Therefore, this method is restricted to a limited number of materials—also among the metals.
Further prior art is known from U.S. Pat. Nos. 5,190,630 A, 4,505,798 A, EP 1 626 432 A1, US 2005/0274610 A1 and WO 03/71579 A1.